1. Field of the Invention
The present invention relates to a method and an apparatus for observing a porous amorphous film and a method and an apparatus for forming the porous amorphous film, more particularly to a method and an apparatus for observing shapes of voids in a porous amorphous film, and a method and an apparatus for forming the porous amorphous film.
2. Description of the Related Art
As semiconductor integrated circuits are getting smaller and faster, wiring capacitance has been a significant issue because it causes wiring delay. An insulation film having low dielectric constant has been focused on as an element which reduces wiring capacitance (parasitic capacity).
A porous insulation film is one of the low dielectric constant film. The porous insulation film has been formed by reducing crystal density of the film. In such the porous insulation film, porosity is inverse proportion to dielectric constant. For obtaining an insulation film having desired dielectric constant, the insulation film should have predetermined appropriate porosity. Measuring topographical characteristics of a porous insulation film such as void""s diameter and porosity is a necessary factor for maintaining and managing the topographical characteristics.
Gas adsorption method (for example, using BET (Brunauer Emmet and Teller""s) formula), observation by scanning electron microscope (SEM) or transmission electron microscope (TEM) are known methods for measuring the topographical characteristics of an porous material.
The gas adsorption technique approximates void""s diameter, porosity, etc. The characteristics as above are calculated based on gas pressure when the gas is condensed in voids with using Kelvin""s capillary tube condensation theorem. In this case, gas including molecules whose occupied cross sectional areas have been known is used.
The results obtained by the gas adsorption method, however, often have errors because the method assumes a cylinder model as the void.
In the observation by SEM, a porous material is scanned by a focused electron beam which is accelerated by voltage of, for example, 10-30 kv to detect secondary electrons emitted from the material""s surface, and image data are prepared based on the detection. Void""s diameter, porosity, etc. of the porous material are measured by observing the image data.
However, it is difficult to observe a void having a diameter of around 10 nm with using SEM, because SEM""s maximum magnification is not so high in such range.
In the observation by TEM, an electron beam which is accelerated by an acceleration voltage of 100-300 kv is irradiated onto a sample which is a slice of porous material. Then, the electron beam transmitted through the sample is enlarged to prepare image data. Void""s diameter, porosity, etc. of the porous material are measured by observing the image data.
Since maximum magnification of TEM is higher than that of SEM, the resultant image data of the TEM observation represents finer image which shows voids in detail. Therefore, more accurate measurement of topographical characteristics of the porous material such as void""s diameter, porosity, etc. is available by the TEM observation.
In a case where a porous insulation film is made of an amorphous material such as a silicon oxide film, available image is unclear even if the TEM is used, because contrast between voids and insulator portions (silicon oxide crystal) in image is low. As a result, it is difficult to distinguish voids from other portions, and to observe shapes of voids in the porous insulation film.
The present invention has been made in consideration of the above, and it is an object of the present invention to provide a method and an apparatus for observing a porous amorphous film by which accurate observation of voids"" shape in the porous amorphous film, and a method and apparatus for forming the porous amorphous film are available.
To achieve the above object, it is provided a method for observing a porous amorphous film comprising:
forming a crystalline thin film on inner surfaces of voids in a porous amorphous film; and
generating image data representing the porous amorphous film having the crystalline thin film with using a transmission electron microscope.